| Abstract
| - When stabilized BH3-THF (BTHF) was added to a mixture of ketone and tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole (MeCBS-ozaxaborolidine, MeCBS) catalyst 1,low enantioselectivities resulted. Several relative rate experiments showed that a borohydridespecies in BTHF catalyzed the nonselective borane reduction of ketones, effectively competing withenantioselective MeCBS reduction of ketones, lowering the overall selectivity of the reaction.Improved enantioselectivities in the reaction are obtained by reversing the mode of addition (ketoneto BTHF and catalyst), lowering the concentration of NaBH4 stabilizer in the BTHF solution (87−95% ee) and increasing the concentration or addition rate of BTHF. Decreased reaction temperatureand increased catalyst loading only slightly improved the selectivity of the reaction. Upon reactionparameter optimization, simultaneous addition of substrate and BTHF to MeCBS catalyst stabilizerresulted in the highest overall enantioselectivities (96% ee) and diminished the effect of theborohydride. Alternatively, the addition of Lewis acids such as BF3-THF to the reaction mixtureeffectively destroyed the NaBH4 stabilizer in BTHF solutions, restoring the enantioselectivity toacceptable levels.
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